Processes for producing chlorine dioxide



Patented Apr. 17, 1945 anssurn 'TorstenGuitailiolsLOrnakoldnik, Sw IAY101949 No Drawing. Application October 19, 1943, Se-

- rial No. 508,859. In Sweden 2, 1843 13 Claims.

The present invention relates to a process for producing chlorine dioxide by reacting a metallic chlorate, e. g.-sodium chlorate, with a gaseous reducing agent, e. g. sulphur dioxide, which process from a technical point of view is simple to carry out and economically favourable, as compared with previously known methods.

According to the invention, this result is ob-' --tained thereby that the gaseous reducing agent is brought to act on the chlorate in an aqueous solution acidified in advance by means of any strong acid, with the exception of halogenic acids, which solution is at least. 2-normal with regard to its content of free acid. To this end. accord-.

ing to an appropriate embodiment of said process, the gaseous reducing agent is added in admixture with an inert gas, e. g. air or nitrogen, in a suitable dilution, for instance at a concentration of 5 to '10 per cent by volume of the gas quantity.

By the process according to the invention there is obtained a chlorine dioxide practically free from chlorine to volumes of Ch per 100 voltimes of 010:) at a high yield, calculated on the quantity of chlorate consumed. Properly carried out, the process will thus give a yield of at least 90%. of the theoretical yield.

Previously it has been proposed U. S. Patent specification No, 2,089,913) to produce chlorine tion or the chlorate into chloride proceeds ac-' cording the formula: V

(1) NaC1Os+3$O-.-+3H:O- NaCl+3HaS04 The reaction proceeds with a considerable development of heat caused by the oxidation of the sulphur dioxideto form sulphuric acid. Only when a certain concentration of the acid has been obtained in the reaction solution, an evolution dioxide (in admixture with chlorine) by reducing an aqueous solution of a metallic chlorateby however, could not outweigh the economical loss ensuing due to the fact that the yield of chlorine dioxide is'low. The U. S. patent. specification does not clearlyv give any figures of yield, but it appears from an example of realisation that the yield of chlorine dioxide only amounts to approx--. imately 60%,.calculated on the quantity of chlorate consumed, which besides has been confirmed by tests carried out.

Investigations carried out by the inventor have shown that the lowv yield" in said known process is principally due to the fact that a largep'ortion of the sulphur dioxide introdueed into the chlorate solution is consumed for the formation of chloride and free sulphuric acid, whereby great losses in chlorate arise. Thls'complete reduc-- of chlorine dioxide begins with successively increasing speed, according to the reaction formula:

2Nec1o=+sor 2c1o=+mism The process has a pronounced autocatalytical course, i. e. it accelerates itself, and after a certain time of reaction the speedof the evolution of C10: reaches a maximum, then remains, during some time, comparatively, constant, whereupon it rapidly sinks at the end of the process.

By reacting the sulphur dioxide with the chlorine. dioxide present in the solution, which reaction may be conceived to proceed according to the reaction formula:

there is forined a further amountpf sulphuric acid and chloride, whereby inevitable losses ensue. However, to all appearances these are of a considerably smaller extentthan those caused by a complete reduction of the'chlorate into chloride.

On account of the successive increase of the concentrations of the chloride and hydrogen ions during the process, conditions arise permitting a development of chlorine according to the schematic reaction formula:

or course, thisformula only intends to indicate stoichiometrically the gross .proportions of'chlorate, chloride and hydrogen ions, which are a condition of the development of chlorine, and

' does not pretend to express exactly the real reaction mechanism, in which certain unstable intermediate products, e. g. HCIO: and HClO are very likely to play an important part.

the chlorine formed may be reduced by the sulphur dioxide accordingto the following reaction formula: (s) -c i+so2+2mo- 2Hc1+rnso4 and, as a result, new amounts of chloride and acid are formed. v 1

As will be evident from the above analysis of the reactions, which is based on detailed experi- 4 mental investigations, extraordinarily complicated conditions exist in the reaction of sulphur dioxide with chlorate solutions, and sources of losses and accompanying low yield of chlorine dioxide are present.

According to the present invention, which is based on the above-mentioned experiences, these losses are avoided to a very great extent due to the fact that the r-casting chlorate solution prior to the reaction is mixed with a certain, sumcient amount of acid, e. g. sulphuric acid, so that a comparatively, high concentration of the acid is obtained. The amount of sulphuric acid added must not be too small, nor too great, in order that a high yield of chlorine dioxide may be obtained. It has proved that a concentration of sulphuric acid or 3.5 to 4 mol/litre (l-normal to.

s-normal) in the finished reaction mixture yields the best results, but of" course the process is not The unstable compound RG: primarily formed reacts, however, very rapidly with HClO; with evolution of chlorine dioxide, C101, according to the formula:

on account of the fact that the chlorate solution already initially has a high concentration oi the acid, the considerable retardation of the evolution of chlorine dioxide is avoided, which retardation sets in when sulphur dioxide is introduced into a neutral or only slightly acid chlorate solution, and causes a productive formation of chloride and sulphuric acid with accompanying loss of chlorate.

An important factor for the favourable carrying out of the process is further the manner in which the gaseous reducing agent, e. g. the sulphur dioxide, is supplied to the acid chlorate solution. It has been found that the concentration should not exceed 10 per cent by volume of of chlorine dioxide is essentially controlled by the amount. of the gaseous reducing agent supplied per unit of time in diluted form, which renders possible an extraordinarily convenient and reliable control of the reaction.

The formation of chloride is very inconsiderabiHnly 4 to 5% of the chlorate is converted into ch.oride at a yield of 90% of ClOs-8lld depends-mainly on a certain unavoidable reduction of the chlorine dioxide by reaction with suiphur dioxide, as already mentioned.

As a result of the low concentration of the chloride in the solution, also the development of chlorine will be extremely inconsiderableat a yield of 90% of chlorine dioxide, calculated on the amount of chlorate consumed, the average volumetric relation ClOslCls in. the evolved gas is equal to 30--and, on the whole, hardly asserts itself as long as the concentration of the chlorate is comparatively high.

According to an appropriate embodiment of the process intended to utllisechemicals present in the solution subsequent to the reaction, e. g. sulphuric acid and sodium sulphate, the concentration of the acid of the solution is increased after the decomposition, e. g. by evaporation and, if desired, addition of a further amount of acid. whereupon the solution is cooled, causing acid sulphate of sodium to crystallise out. After the last-mentioned salt has been separated, the concentration of the acid of the solution is adusted to a suitable value, whereupon the solution is .ready to be mixed with fresh chlorate solution and thus be re-introduced into the process.

The process according to the invention may be carried out continuouslyas well as periodically. The contact between the acid chlorate solutionand the gaseous'reducing agent may also be effected in diiferentmanners, e. g. by permitting the ilnely divided gas to .pass through the acid chlorate solution which is kept in vigorous motion by mechanical stirring.

The manner of carrying out the process according to the invention is further described below by means of the following example of realisation.

Example 100 mol 14.5-normal sulphuric acid, prepared a from 4 volumes of concentrated H2804 and 6 SO: in the inert gas mixture and that the most favourable results are obtained at low contents of $02, for instance 5 per cent by volume. A good contact between the incoming gas and the liquid reaction mixture is also of essential importance and is most easily obtained by the atomisation of the gas and a vigorous stirring of the liquid.

Inthismanneritispossibletocarryoutthe reduction of the chlorate into chlorine dioxide with a very good yield and to obtain a chlorine dioxide almost quite free from chlorine and diluted with the inert gas, e. g. air or nitrogen. to such a concentration, for instance 5 per cent by volume, that there will be no risk of explosion.

The process takes place under the abovementioned conditions with very inconsiderable evolution of heat and may to great advantage be carried out at approximately ordinary temperature. for imtance 10 to 40 0., preferably 30 to 25 C. Thus, no measures need be taken for heating or cooling the reaction solution. The rapidity of the reaction and thus thespeed of the evolution volumes of water, was mixed. with mol of a 48% by weight solution of sodium chlorate (containlng 65 gr. NaClOa).

This mixture was introduced into a reaction vessel consisting of a globular flask having a volume of about 0.5 litre and provided with three necks. A mechanically driven stirrer of glass was introduced through the middle one; through the two others respectively supply and discharge of gas was eilected.

An even flow of nitrogen having a velocity of 40 litres per hour was mixed with a flow of sulphur dioxide having a velocity of 2 litres per hour. This gas mixture was introduced (at room temperature) into the acid chlorate solution, while the solution all the time was subjected to a very vigorous stirring. Hereby there was obtained in the leaving flow of nitrogen 37.7 gr. of chlorine dioxide at a concentration practically constant during the greater part of the time of test and amounting to about 5 per cent by volume, corresponding to a yield of 92%. The chlorine dioxide evolved was. duringthe greater part of the process, almost quite free from chlorine (ClO's:Cls=50 to 100) and appreciable amounts or chlorine could be identified only .towardsthe end.

. Alter complete de-aeration at elevated temperature, the remaining reaction solution: was analysed, when inter alia thexi'ollowing results were obtained: w.. e Concentration of chlorate;

Concentration of chloride: 0.1-n0rmal Concentration of acid: 8.4-normal Having now particularly described the nature or my invention and the manner of its operation what I claim is: I

1. In a'process for producing chlorine dioxide by reacting a metallic chlorate with a gaseous reducing agent, the step of bringing the gaseous reducing. agent to act upon the chlorate in an aqueous solution acidified in advance by means of any strong acid, with the exception 0! halogenic acids, said solution being at least 2-normai with regard to its content of tree-acid.

2. In a process for producing chlorine dioxide by reacting an alkali metal chlorate with gasedioxideto acton the chlorate in an aqueous solution in advanceby'me'ansot any strong acid, with or halogenicacids, said aohition being at least a-normal with regard to its oontentoi free acid and carrying out the reaction t! temper ture of loto 40 C.

8. In a processerorl producing chlorine dioxide by reacting sodium chlorate with gaseous sulphur dioxide the steps orbringing the gaseous sulphur dioxide to act on the chlorate in an aqueous solution acidified in advance by means or any strong acid, with the exception of halogenic acids, said solution'being at least -normal' with regard to its content free acid, adding the gaseous sulphur dioxide in admixture with an inert gas of the group consisting of air and nitrogen at a concentration amounting to about 5 to per cent'by volume oi'lthe gas quantity and carrying out the reaction its temperature of 10 to 40 C.

ous sulphur dioxide, the step of bringing the gaseous sulphur dioxide to act on said alkali metal chlorate in an aqueous solution acidified in advance by means or any strong acid, with the exception of halogenic acids, said solution being at least 2-normal with regard to its content oi free acid.

3. In a process for producing chlorine dioxide by reacting sodium chlorate with gaseous sulphur dioxide, the step of bringing the gaseous sulphur dioxide to act on the chlorate in an aqueous solution acidified in advance by means of any strong acid, with the exception of halogenic acids, said solution being at least 2-nor-mal with regard to its content of free acid.

4. In a process for producing chlorine dioxide by reacting sodium chlorate with gaseous sulphur dioxide the steps of bringing the gaseous sulphur dioxide to act on the chlorate in an aqueous solution acidified in advance by means of any strong acid, with the exception of halogenic acids, said solution being at least 2-normal with regard to its content of free acid, and adding the gaseous sulphur dioxide in admixture with an inert gas 01' the group consisting of air and nitrogen.

5. In a process for producing chlorine dioxide by reacting sodium chlorate with gaseoussulphur 50 dioxide the step of bringing the gaseous sulphur dioxide to act on the chlorate in an aqueou solution acidified in advance by means or any strong acid, with the exception of halogenic acids, the

acid used in acidification of the chlorate solution 58 consisting ofsulphuric acid and being added in such an amount that the concentration of the acid in the reaction mixture obtained is 6 to 9- normal.

6. In a process for producing chlorine dioxide do 9. In a process for producing chlorine dioxide by reacting sodium chlorate with gaseous sulphur dioxide the steps of bringing the gaseous sulphur dioxide to act on the chlorate in an aqueous solution acidified in advance by means of any strong sciii, with the exceptionoi halogenic acids, said solution being at least iii-normal with regard to its 'content or acid and recovering acid sulphate of sodium, 7

N&HBO4.H2O

dioxide the steps of bringing the gaseous sulphur dioxide to act on the chlorate in an aqueous solution acidified in advance by means or any strong acid, with the exception of halogenic acids, said solution being at least 2-normalwith regard to its content of free acid, and adding the sulphur dioxide in admixture with an inert gas of the group consisting of air and nitrogen at a conand nitrogen at a concentration of about 5 to 10 per cent by volume 01' the gas quantity, the acid used in acidification of the chlorate solution consisting of sulphuric acid and being added in such an amount that the concentration 01 the acid in the reactionmixture obtained is 6 to 9-normal.

7. In a process for producing chlorine-dioxide by reacting sodium chlorate with gaseous sulp centration of about 5 to 10 per cent by volume of the gas quantity, and recovering acid sulphate of sodium,

NaHSOaHaO from the consumed reaction solution by evaporation and cooling to a suitable mperature for crystallizing out sodium bisulphate, whereupon the remaining solution is used for renewed reaction with chlorate.

11. In a process for producing chlorine dioxide by reacting sodium chlorate with gaseous sulphur dioxide,the steps of bringin the sulphur dioxide to act upon said sodium chlorate in an aqueous solution acidified in advance by means or any strong acid, with the exception oi halogenic acids, said solution being at least a-normahwith regard to its content orthe acid,-- and recoverin acid sulphate oi sodium, v 7

NaHBOaHaO from the consumed reaction solution by evaporation and addition of sulphuric acid and cooling to a suitable temperature for crystallizing out sodium bisulphate, whereupon the remaining solution isused forthe renewed reaction with chlorate.

12. In a process for producing chlorine dioxide by reacting sodium, chlorate with gaseous suid d the ps oi bringing the gaseous sulp u 7s phur dioxide the step or bringing the gaseous 4 a,a1a',aao I m nmmetomouthumnhumma -ounolutton acldmcdmadnncabymcamouny 1:. PM for much: chlorine mm,-

which comm-he's sulphur dion: solution am in advance by lddmon of aulphurlc m in such s quantity tint 1010mtlonhat-lcutl-mnlwlthrcnrdtoltlcontcnt or (reg mlphm-tc acid.

mas-nu GUST" 'BOLST. 

